Electrocatalytic hydrogenation process

ABSTRACT

Steroids having a multiple carbon-carbon bond which is isolated or conjugated with an aromatic ring and which are hydrogenatable to a mixture of stereoisomers are stereo-specifically and selectively reduced by electrocatalytic hydrogenation in an acidic solvent employing a cathode whose surface is a finely divided metal hydrogenation catalyst, such as Raney nickel or palladium.

United States Patent Junghans Dec. 9, 1975 ELECTROCATALYTIC HYDROGENATION PROCESS lnventor: Klaus Junghans, Berlin, Germany Assignee; Schering Aktiengesellschaft, Berlin & Bergkamen, Germany Filed: May 2, 1974 App]. No.: 466,172

Foreign Application Priority Data May 4, 1973 Germany 232309l US. Cl 204/73 R; 204/72; 260/397 Int. Cl. C25B 3/04; C07C l67/l2:

C07C l69/00 Field of Search 204/72, 73 R References Cited UNITED STATES PATENTS 9/l969 Schmidt et a1 t. 424/243 3,8l4,7ll 6/l974 Eloy et al 260/287 R Primary Examiner-F. C. Edmundson Attorney, Agent. or Firm-Millen, Raptes & White ABSTRACT 9 Claims, No Drawings ELECTROCATALYTIC HYDROGENATION PROCESS BACKGROUND OF THE INVENTION This invention relates to a process for the stereospecific electrocatalytic hydrogenation of unsaturated steroids.

The stereoselective reduction of unsaturated organic compounds, including steroid compounds, by chemical catalytic hydrogenation using finely divided metal catalysts has been known for a long time.

The reduction by electrolytic hydrogenation employing finely divided, catalytically active metal cathodes for the reduction of organic compounds having a simple structure, such as ethylene, has also been described. (J. Chem. Soc. Farad. Trans. 590 [I972] 355) or maleic acid (C.A. 72 [1970] 382l9c; ibid., 74 [1971] 27502e; ibid. 74 [1971] l245lu).

It is also known that the electrolytic reduction employing conventional cathodes of unsaturated steroids occurs by trans-hydrogenation. See US. Pat. No. 3,720,694 and German Unexamined Laid-Open Application DOS 2,029,415.

The process of this invention is directed to selectively hydrogenating unsaturated steroids having a complicated structure to stereochemically uniform compounds.

SUMMARY OF THE INVENTION According to this invention, steroid compounds having at least one carbon-carbon multiple bond which is isolated or conjugated with an aromatic ring is electrocatalytically hydrogenated in an acidic solution employing cathode whose surface is a finely divided metal hydrogenation catalyst.

DETAILED DISCUSSION Starting steroids for the process of this invention are those steroids having a complicated structure, i.e., steroids having at least one carbon-carbon multiple bond which is either isolated or conjugated with an aromatic ring and which is hydrogenatable to a mixture of stereoisomers, e.g., when hydrogenated in a conventional manner with zinc dust and acetic acid or with hydrogen and palladium on charcoal catalyst.

It was surprising that the electrocatalytic hydrogenation of simply structured organic compounds could be transferred to unsaturated organic compounds having a complicated structure, such as sterically hindered steroids, and that the carbon-carbon multiple bonds thereof would be stereospecifically reduced.

It was also surprising, especially in di-tertiary carboncarbon bonds, that the transfer of hydrogen to the steroid molecule occurs exclusively by cis-hydrogenation. In case of one or more possible centers of asymmetry, only one stereoisomer is formed in every instance.

The process of this invention is conducted by depositing the hydrogen required for the hydrogenation electrolytically on a cathode whose surface is a finely divided metal hydrogenation catalyst. (See the scientific publications cited above whose disclosures are incorporated herein by reference.) All that is required is that the metal be in finely divided hydrogenation catalytically active form. Suitable metals are those of Subgroup VIII of the periodic table, e.g., iron, cobalt, nickel, palladium, osminum, iridium and platinum, es-

pecially suitable are palladium, nickel or platinum. The finely divided metal is applied in finely divided form on an ainert cathode support of any desired density in a convention manner; this support can be formed, for example, of copper, gold, ABS (acrylonitrile-butadienestyrene) or other solid synthetic resin, platinum or pal ladium. Its shape is not important. At the laboratory scale, a strip or a plate can be used. Preferred cathodes are Raney nickel and palladinated platinum.

The specific anode material employed has no effect on the hydrogenation of this invention, so long as it is stable under the conditions of the electrolysis. Preferred electrodes are platinum, palladium, platinated titanium, graphite, ferrite, manganese(lV)-dioxide and lead(IV)-dioxide.

The term finely divided" metal as used herein means that the catalytically active metal deposited on the surface of the cathode support exhibits an effective surface area at least ten times greater than the geometric surface area of the cathode.

The starting steroid is reduced in an acidic solution, i.e., having a pH of less than 7, preferably from about 5 to 0. Suitable as the solvents are all the solvents conventionally used for hydrogenations. Preferred are solvents which are miscible with water, e.g., lower alcohols of l8 carbon atoms, e.g., methanol, ethanol, propanol, isopropanol and butanol, polyhydric alcohols, e.g., ethylene glycol, propylene glycol and glycerin, dialkyl ethers, e.g., diethyl ether, cyclic ethers, e.g., tetrahydrofuran and dioxane; chlorinated hydrocarbons, e.g., chloroform and dichloroethylene; acid derivatives, e.g., dimethylformamide and acetonitrile; and, last but not least, water by itself or in a mixture with the aforementioned solvents, or mixtures of these solvents, as required to dissolve the starting steroid.

The starting steroid can be dissolved in the preacidified solvent or the pH of the solvent can be rendered acidic after the steroid is dissolved therein.

A preferred reaction solvent is a lower-alkanol, alone or in admixture with a cyclic ether, mixed, in a volume ratio of from about l0:l to about l:1, with aqueous, e.g., about l-IO percent hydrochloric, hydrobromic or sulfuric acid or with concentrated hydrochloric or sulfuric acid.

The electrolytic hydrogenation can be conducted from below room temperature to the boiling point of the reaction mixture, e.g., from about 20-80 C. The reaction is continued preferably until all of the starting steroid has been hydrogenated, usually for about l-24 hours, e.g., 2-8 hours.

The solvent is preferably acidified with a mineral acid, e.g., hydrochloric acid, perchloric acid, phosphoric acid, sulfuric acid or fluoboric acid, which serves simultaneously as the electrolyte. Organic acids, e.g., acetic acid, trifluoroacetic acid and propionic acid, are suitable. The amount of acid added can be varied within wide limits and has no effect on the hydrogenation per se. The acid, e.g., acetic acid, for example, can optionally also be employed as the hydrogenation solvent. It will be apparent to those skilled in the art that sufficient electrolyte must be present in the solution to provide the desired current density.

The electrolysis is advantageously conducted in a divided cell, i.e., one in which the anode and cathode chambers are separated from each other by a porous material, such as, for example, clay, porous glass, or an ion exchanger membrane.

A divided cell has the advantage that the electrolytic reaction which occurs at the cathode is not influenced by the reaction at the anode.

The electrolysis is effected according to conventional methods. It is not critical that the current or the potential be maintained constant. However, it is advantageous to operate at a constant current. The magnitude of the amperage can be varied within wide limits, e.g., from 1 mA to 50 A, depending on the geometric surface of the electrodes and the desired rate of electrolysis. However, it is desirable to employ a current density of 0.01 1O A/cm, based on the geometric surface of the electrode. Voltage can vary widely, e.g., from 0,5 to 400, preferably from 2 50 volts.

The electrolysis is preferably conducted at room temperature. However, it is also possible to effect the electrolysis at lower or elevated temperatures, even at the boiling temperature of the reaction mixture.

The starting steroids can have one or more carboncarbon multiple bonds. Examples of such isolated multiple bonds are endocyclic double bonds, for example, in the l( l)-, 4(5)-, 5(10)-, 5(6)-, 8(9)-,9(l1)- and/or l4-position. The multiple bonds can also be exocyclic, e.g., as a methylene group in, for example, the 1-, 6- and/or l6-position, or as an alkinyl group, e.g., in the l7-position. If the steroid molecule is substituted in the 1-position, e.g., with a methyl group, a A'-double bond is likewise stereospecifically hydrogenated in accordance with the process of this invention. If more than one such multiple bond is present in the steroid molecule, they are hydrogenated independently of one another.

The process of this invention has the advantage that the aromatic A-ring which is sometimes present in steroids is not concomitantly reduced. A carbonyl group, which may be present in the steroid molecule also is not attacked. However, isolated double bonds, such as the A-, 11 11 A-, A- or A -double bond, which do not lead to a center of asymmetry, are also reduced.

Although the process of this invention is of particular usefulness in the stereospecific hydrogenation of unsaturated steroids of the pregnane and androstane series, it will be obvious to one skilled in the art that the exact nature of the side chain at the 17- and/or 20-position is not critical and the process is also applicable to unsaturated steroids of, e.g., the coprastane, cholestane, cholane, norcholane, bis-norcholane, etiocholane, stigmastane and sitostane series. A preferred class of starting steroids are those having an aromatic A-ring and preferably also a free, etherified or esterified 3-hydroxy group, or an isolated A-double bond, preferably in combination with a 3-keto group or a free, esterified or etherified 3-hydroxy group. The angular methyl groups at the 18- and/or l9-position can be present or absent or replaced by another lower-alkyl group, e.g. ethyl. A methyl group can also be present at any of the available positions of the steroid nucleus, e.g., i, 2, 6, 7, 9, 12 and/or 16 positions. Functional groups, e.g., hydroxy or keto, can be present at, for example, the 6, 7, ll, l2, 16 and/or 20-position, hydroxy or acyloxy, preferably acetoxy, can be present, e.g., at the 6-, 7-, 17- and/or 20- positions. A halogen atom, e.g., Cl or F, can be present at, e.g., the l 2, 6, 7, 9 and/or 16-positions. Other functional groups, e.g., oxide, can also be present in the molecule, e.g., in the 16,17-position.

An especially preferred class of starting steroids are A-ring aromatic 3,17-oxygenated steroids of the estrane series having at least one isolated multiple bond,

e.g., a double bond, in the 8, 9(11), l4-position of the nucleus, and/or a l6-methylene or 17-alkynyl group. The oxygen function in the 3-position is preferably hydroxy, alkoxy, e.g., of 1-4 carbon atoms, or acyloxy, preferably alkanoyloxy, e.g., of 2-8 carbon atoms, and the oxygen function in the l7-position is preferably B- hydroxy, B-acyloxy, keto, flalkynyl-Ha-hydroxy, B- acetyl-a-hydroxy, B-acetyl-a-acyloxy, B-hydroxy-acetyl-a-hydroxy, B-acyloxyacetyl-a-hydroxy or B-acyloxyacetyl-a-acyloxy. It will be obvious that when the starting steroid bears an acyloxy group, the exact nature thereof is not critical and can be aliphatic, aromatic, alicyclic or heterocyclic and can be free ofother functional groups or can bear one or more such groups, e.g., hydroxy, carboxy, amido, cyano, halo, etc. Specific examples are those wherein the acyl group is that of an alkanoic acid of 1-18, preferably 2-l2 carbon atoms, e.g., acetic and undecylic, ofa cycloalkanoic or cycloalkylalkanoic acid, e.g., cyclohexyl-carboxylic acid B-cyclopentyl-propionic acid, of a carbocyclic aryl acid of 612 carbon atoms and having l-2 separate or fused rings, e.g., benzoic, p-toluic, a-naphthoic, or a carbocyclic aralkyl acid of 7-12 carbon atoms, e.g., phenylacetic, of a heterocyclic acid of 5-12 carbon atoms and l-2 heteroatoms, e.g., pyridine-Z-carboxylic acid, thiophene-Z-carboxylic acid, furane-Z-carboxylic acid, of a polybasic acid, containing 2-8 carbon atoms, e.g., succinic acid, or of an alkanoic or carbocyclic acid bearing a substituent, e.g., halo, hydroxy, alkoxy, acyloxy, e.g., trichloroacetic, lactic, citric, salicyclic, phenoxyacetic acid. The acyl radical can also be that of a sulfonic or other acyloxy acid, e.g., benzenesulfonic, p-toluenesulfonic, methanesulfonic and ethanesulfonic acid.

The process of this invention has the advantage that the hydrogenation can be conducted under normal pressure with a low expenditure in appartus and additionally can be designed as a continuous process by using electrolytical cells of the tube reactor or filter pressing type (see, for reference, Electro-Organic Chemical Processing by Charles L. Mantell, Noyes Dev. Corp., Parkridge, N. 1., U.S.A., 1968 or Organic Electrochemistry by Manuel M. Baizer, Marcel Dekker Inc., N.Y., 1973).

The compounds producible according to this invention are intermediates for the preparation of valuable pharmaceutical agents. Thus, l3-ethyl-3-methoxy-8- isogona-l ,3,S( l0)-trien-l7,B-ol (U-S.P. 3,407,217), for example, is obtained from 13-ethyl-3-methoxy-8- isogona-l,3,5(l0)-trien-l7B-one by reduction of the l7-oxo group with sodium borohydride in methanol.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever.

EXAMPLE 1 1.0 g. of 3-methoxy-l,3,5(l0),8-estratetraen-l7B-ol is dissolved in ml. of ethanol and electrolyzed at room temperature in a divided cell (glass diaphragm), after the addition of 5 ml. of concentrated sulfuric acid, for 6 hours at 0.75 A on a Raney nickel electrode. After the reaction mixture has been worked up, 1.0 g. of a colorless reaction product is isolated which, according to analysis by gas chromatography, contains, in

addition to 0.15 g. of starting material, 0.85 g. of 3- methoxy-1,3 ,5(10)-8a-estratrien-l713-01, m.p. l-102 C.

EXAMPLE 2 0.5 g. of 3-methoxy-1,3,5(l0), 8-estratetraen-17B-ol is dissolved in 100 ml. of isopropanol and 20 ml. of tetrahydrofuran and, after the addition of 20 ml. of 15 percent perchloric acid was electrolyzed for 3 hours at about 1.0A at room temperature on a palladinised platinum electrode in a cell divided by a cation exchanger. After the reaction mixture has been worked up, the product is 0.48 g. of Sa-estradiol methyl ether in the form of colorless crystals, m.p. 65 C. (methanol).

EXAMPLE 3 1.0 g. of 3-methoxy-l,3,5(l0), 9(11)-estratetraen- 1713-01 in 80 ml. of ethanol is heated, together with 10 ml. of concentrated hydrochloric acid, to the boiling point and electrolyzed at the boiling temperature of the mixture (about 80 C.) for 2 hours on a palladinised platinum electrode in a cell divided by a cation exchanger. After the mixture has been worked up, 0.95 g. of 8B-estradiol methyl ether is obtained, melting after chromatography and recrystallization in benzenepetroleum ether, at 100-101 C.

EXAMPLE 4 0.05 g. of 3-methoxy-l,3,5(10),6,8,l4-estrahexaen- 175-01 is electrolyzed in 7.5 ml. of ethanol and 2.5 ml. of 10 percent sulfuric acid for 6 hours at room temperature in a divided cell on a palladinised palladium electrode. After working the reaction mixture up as described in Example 1, the product is 3-methoxy- 1,3,5(10),6,8-estrapentaen-l4a-H-173-01, m.p. 144-145 C.

EXAMPLE 5 0.5 g. of 3-methoxy-l8-methyl-l,3,5(10),8-estratetraen-l7-one is electrolyzed in 100 ml. of propanol and 30 ml. of dioxane with 20 ml. of percent hydrobromic acid at room temperature for 6 hours on a palladinised copper electrode in a cell divided by a clay diaphragm. After the mixture has been worked up, the yield is 0.48 g. of 3-methoxy-l8-methyl-1,3,5(10)-8aestratrien-l 7-one, m.p. 9293 C., in addition to 0.065 g. of unreacted starting material.

EXAMPLE 6 1.0 of 3-methoxy-l7B-acetoxy-18-methyl- 1,3,5(10),8,14-estrapentaene in 150 ml. of ethanol is electrolyzed for 7 hours at room temperature with ml. of 10 percent sulfuric acid in a cell divided by a glass frit material on a palladinised gold electrode. After the reaction mixture has been worked up in accordance with Example 1, the product is 0.61 g. of 3- methoxy-l7B-acetoxy-l8-methyl-1,3,5(l0)-8a,14aestratriene, in addition to 0.36 g. of 3-methoxy-18- methyl-1,3,5(10)-8a,l4a-estratrien-17fi-ol.

EXAMPLE 7 0.1 g. of 3B-hydroxy-l7B-benzoyloxy-androst-S-ene is electrolyzed in 80 ml. of ethanol and 20 ml. of tetrahydrofuran with 50 ml. of 5 percent sulfuric acid at 80 C. for 2.5 hours in a cell divided by a cation exchanger diaphragm on a palladinised platinum electrode. The reaction product consists, according to analysis by gas chromatography and NMR, of 0.09 g. of BB-hydroxyl7B-benzoyloxy-5a-androstane in addition to 0.01 g of 5a-androstane'3,l7B-diol.

EXAMPLE 8 0.05 g. of 3-methoxy-l7B-hydroxy-l6-methylene- 1,3,5(10)-estratriene in 50 ml. of ethanol and 10 ml. of tetrahydrofuran is electrolyzed with 20 ml. of 10% hydrobromic acid for 2.5 hours in a cell divided by a cation exchanger diaphragm on a palladinised palladium electrode. After working up the reaction mixture analogously to Example 1, 0,04 g. of 3-methoxy-l 7B- hydroxy-16B-methyl-1,3,5( l0)estratriene is obtained, m.p. 116 C.

EXAMPLE 9 0.10 g. of 3-methoxy-17B-hydroxy-l7a-ethinyll,3,S(lO)-estratriene is electrolyzed in 25 ml. of etha n01 and 20 ml. of tetrahydrofuran with 5 ml. of 10 percent sulfuric acid for 3 hours in a cell divided by a cation exchanger diaphragm on a palladinised palladium electrode. After the reaction mixture has been worked up, 0.08 g. of 3methoxy-l7fi-hydroxy-l7a-ethyl- 1,3,5(10)-estratriene is obtained, m.p. 180 C.

EXAMPLE 10 0.1 g. of a mixture, approximately 1:1, of 6-ethoxycarbonylmethyl- 1 ,3,5( 10 ),6-estratetraene-3 l 7B-diol and 6-ethoxycarbonylmethy1ene'l ,3,5( l0)-estratriene- 3,17B-diol in 25 ml. of ethanol and 10 ml. of tetrahy drofuran is electrolyzed with 10 ml. of 1 percent sulfuric acid for 5 hours on a palladinised platinum electrode in a cell divided by a glass frit material. After the reaction mixture has been worked up. 0.07 g. of 6B- ethoxycarbonylmethyl-1,3,5( 10 )-estratriene-3 l die] is isolated.

NMR spectrum: 0.82 (s, 18 H); 1.28 (t, .l 7 Hz; CH -CH,); 2.60 (m, CH CO); 3.35 (m, w 20 Hz, 6H); 3.75 (t, J 7.5 Hz, 17H); 4.17 (q, 1 7 Hz, CH -CH,O); 6.64 (m, W 12 Hz, 2H and 4H); 7.11 (d, J 9 Hz, 1H).

EXAMPLE 11 0.1 g. of 3-hydroxy-l7B-hydroxy-17a (butin-1-yl)- 1,3,5(10)-estratriene is electrolyzed in 25 ml. of ethano! and 20 m1. of tetrahydrofuran with 5 ml. of 10 percent sulfuric acid in a cell divided by a cation exchanger diaphragm on a palladinised platinum electrode until 1 equivalent of hydrogen has been absorbed. After the reaction mixture has been worked up, 0.09 g. of 3-hydroxy-l7fl-hydroxy-l7a(cis-buten-lyl)- 1,3,5(10)estratriene is obtained, m.p. 120 C.

NMR spectrum: 5.45; 5.60 (.l= 9 Hz -CH=CH); 2.2 (J 7 Hz, C1-1 CH).

The preceding examples can be repeated with similar success by substituting the generically and specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

What is claimed is:

l. A process for the stereospecific selective electrolytic hydrogenation of a carbon-carbon, non-aromatic multiple bond of an unsaturated steroid having an aromatic ring and which is reducible to a mixture of stereoisomers, while leaving the aromatic ring intact, which comprises subjecting an acidic solution of the unsaturated steroid to electrolysis in a divided cell employing a cathode whose surface is a finely divided metal hydrogenation catalyst until the carbon-carbon multiple bond is hydrogenated.

2. A process according to claim 1 wherein the finely divided metal catalyst is a metal of Subgroup VIII of the periodic table.

3. A process according to claim 1 wherein the finely divided metal catalyst is nickel, platinum or palladium.

4. A process according to claim 1 wherein the steroid has an aromatic A-ring and the multiple bond is a A, A, A or A"-double bond.

5. A process according to claim 1 wherein the steroid has an aromatic A-ring and the multiple bond is a 17- alkynyl group.

6. A process according to claim 1 wherein the steroid has an aromatic A-ring and the multiple bond is a 16- methylene group.

7. A process according to claim 1 wherein the starting steroid is an unsaturated steroid of the estrane series having an aromatic A-ring, the finely divided metal catalyst is nickel, platinum or palladium and the reaction is conducted in an aqueous organic solvent acidified with a mineral acid.

8. A process according to claim 1 wherein the cathode is catalytically inert platinum as support coated with finely divided palladium in catalytically active form.

9. A process according to claim 7 wherein the cathode is catalytically inert platinum as support coated with finely divided palladium in catalytically active form.

* a: s =t 

1. A PROCESS FOR THE STEREOSPECIFIC SELECTIVE ELECTROLYTIC HYDROGENATION OF A CARBON-CARBON, NON-AROMATIC MULTIPLE BOND OF AN UNSATURATED STERIOD HAVING AN AROMATIC RING AND WHICH IS REDUCIBLE TO A MIXTURE OF STEROISOMERS, WHILE LEAVING THE AROMATIC RING INTACT, WHICH COMPRISES SUBJECTING AN ACIDIC SOLUTION OF THE UNSATURATED STEROID TO ELECTROLYSIS IN A DIVIDED CELL EMPLOYING A CATHODE WHOSE SURFACE IS A FINELY DIVIDED METAL HYDROGENATION CATALYST UNTIL THE CARBON-CARBON MULTIPLE BOND IS HYDROGENATED.
 2. A process according to claim 1 wherein the finely divided metal catalyst is a metal of Subgroup VIII of the periodic table.
 3. A process according to claim 1 wherein the finely divided metal catalyst is nickel, platinum or palladium.
 4. A process according to claim 1 wherein the steroid has an aromatic A-ring and the multiple bond is a Delta 6, Delta 8, Delta 9(11) or Delta 14-double bond.
 5. A process according to claim 1 wherein the steroid has an aromatic A-ring and the multiple bond is a 17-alkynyl group.
 6. A process according to claim 1 wherein the steroid has an aromatic A-ring and the multiple bond is a 16-methylene group.
 7. A process accordinG to claim 1 wherein the starting steroid is an unsaturated steroid of the estrane series having an aromatic A-ring, the finely divided metal catalyst is nickel, platinum or palladium and the reaction is conducted in an aqueous organic solvent acidified with a mineral acid.
 8. A process according to claim 1 wherein the cathode is catalytically inert platinum as support coated with finely divided palladium in catalytically active form.
 9. A process according to claim 7 wherein the cathode is catalytically inert platinum as support coated with finely divided palladium in catalytically active form. 